The presence of common-mode signals in instrumentation systems that are primarily interested in monitoring differential-mode signals is a common phenomenon. Typical examples of a differential-mode signal analyzer include an electrocardiograph ("ECG") monitoring system or a defibrillator system. Electrodes of these system are placed advantageously on the torso of a patient such that the electrical signals generated by the heart induce a differential signal across the electrodes. These differential-mode signals are of interest because they give the diagnostician an accurate indication of the state of the patient's heart (e.g. normal beat pattern versus ventricular fibrillation).
As is well known in the art, common-mode signals (i.e. signals that appear simultaneously upon both electrodes with essentially equal magnitude, frequency, and phase) are superimposed upon the differential-mode signals of interest (i.e. those generated by the heart) and are sometimes converted by the system into differential-mode signals themselves. As discussed in commonly-assigned and co-filed patent application Ser. No. 08/398,377 (entitled "COMMON MODE SIGNAL AND CIRCUIT FAULT DETECTION IN DIFFERENTIAL SIGNAL DETECTORS", filed March 3, 1995 by Leyde et al. and hereby incorporated by reference), this conversion may lead to the ultimate corruption of the differential-mode signals of interest and, in the case of a defibrillator, may lead to a potentially harmful misdiagnosis of the patient's true heart condition.
Because the possibility of misdiagnosis has potentially serious consequences, several attempts have been made to deal with the problem of common-mode conversion. These efforts have, by and large, been concerned with either the elimination or suppression of common-mode signals. By reducing common-mode signals, the contribution of their effects on the composite signal are similarly reduced.
The reduction of common-mode signals has taken several forms. The first common method is capacitance reduction. As is well known in the art, common mode voltages induce common mode currents inversely proportional to the total impedance around the loop between the patient, the system, and the common mode voltage sources. To reduce common mode currents, this impedance is made as large as possible by minimizing the capacitance between the system and its cables to the outside world.
Nevertheless, capacitance minimization has its limitations. Circuits and cabling occupy certain minimum physical areas, and capacitance can only be reduced by increasing the distance from these circuits to outside references. Outside references may be the earth, or objects outside the instrument, or may even be other parts of the same instrument that have different potential references.
For example, many medical instruments maintain "isolated" circuits connected to patients for safety reasons. These circuits maintain a local potential reference not electrically connected to other references in order to reduce accidental electrical injuries. In these cases, reducing the capacitance to such "isolated" circuits means that spacing must be maximized within the instruments between the isolated circuits and other portions of the instrument, the instrument enclosure, or objects in the outside world. However, it is also important to limit the physical size of instrumentation, so that increasing available spacing has practical limitations as a means of limiting common mode currents.
A second major effort to reduce common mode currents is shielding. In this case, shields are equipotential surfaces such as metal enclosures, that are employed to block the entry of electromagnetic fields into instruments and cabling. Such fields may originate, for example, from power lines, radio transmitters, or nearby moving charged objects and may induce common mode currents in circuits they encounter.
However, instrument shielding does not include the patient--a major source of common-mode coupling. The shielding of the instrumentation system thus does nothing to prevent the presentation of large common-mode sources at electrode connections, after which common-to-differential mode conversion proceeds without inhibition. Shielding can, in fact, make matters worse by increasing capacitance between the instrument ground and earth ground, thus facilitating common-mode current flow.
Closely tailored to the inadequacies of shielding, a third common-mode signal reduction method is the use of extra electrodes. In some systems, a third electrode is attached to the patient and connected to the instrument potential reference in an attempt to shunt common-mode currents around the differential electrode leads. Unfortunately, even this third electrode has its own series impedance. Thus, common mode currents will divide between the differential input leads and the third electrode connection. This results in a reduction--but not elimination--of common mode currents in the differential input leads. Also, the addition of a third electrode adds complication to circuitry that minimally requires only two patient electrodes. A fourth method for reducing common-mode signals is filtering. Some common-mode signals, especially at low frequencies (e.g. below 1 Hz) or at power line frequencies, lie outside the normal passband desired for ECG signals (usually between 1-40 Hertz) and thus the composite signal can be improved somewhat by passband filtering. Nevertheless, much of the energy in both common-mode artifacts and ECG signals occupy the same part of the spectrum, which makes attempts to remove all of the common-mode signal futile. Many time-varying fields encountered in patient treatment fall into the normal ECG passband and have time characteristics that are particularly confusing.
As mentioned above, none of these above-described methods for dealing with the presence of common-mode signal completely eliminate the effects of converted common-to-differential mode signal. Thus, the potential for misdiagnosis is still a very real and serious possibility--even after these above suppression techniques have been tried.
Thus, there is a need for a way to effectively deal with the effects of common-mode signal even after suppression of these signals has been attempted. More specifically, there is a need to analyze the composite signal to avoid the possibility of a serious misdiagnosis of patient's condition.